Electro-hydraulic actuators can be used as signal generators in general hydraulic applications. Of particular interest is the use of an EH actuator in conjunction with a fluidic mud-pulse telemetry transmittor. Mud-pulse telemetry is a technique for transmitting information from the bottom of a well bore to a position at the top of the well where the information may be utilized to control the drilling operation. Sensors located near the drill bit provide electrically coded signals representative of conditions such as temperature, pressure, etc. existing at the bottom of the wellbore. These signals are applied to an EH actuator which controls the operation of a fluidic pulsing device. Pulses are generated in the drilling fluid which flows through the drill string, and may be detected by suitable transducers located at a convenient position at the well head. The coded information represented by the pulses may be interpreted by the drill string operator for use in controlling the drilling operation. The operation of a mud-pulse telemetry system is more particularly described in U.S. Pat. No. 4,276,943 issued July 7, 1981 to Holmes and in U.S. Pat. No. 4,323,991 issued Apr. 6, 1982 to Holmes et al., the disclosures of which are incorporated herein by reference.
In most drilling operations pressures at the drill bit are extremely high, generally on the order of 20,000 psi. Electro-hydraulic actuators currently known are incapable of operating efficiently at such high pressures. Generally, a great deal of power must be provided to the actuator in order to counteract the high pressure and provide a control impulse. This is troublesome as power at the drill bit is generally limited, normally being provided by batteries or a low powered mud turbine. Also, the tremendous pressures encountered by the actuator normally slow the actuators response time to a rate which is insufficient to transmit adequate amounts of data to the drill bit operator.
Accordingly, it is an object of this invention to provide an EH actuator which is capable of operation at very high bore hole pressures with only minimal electrical power.
It is an object of the invention to provide such an actuator having a rapid response rate at high pressures enabling the device to transmit large quantities of information as required in a mud-pulse telemetry system.
It is yet another object of the invention to provide an EH actuator which is durable when operating under very high pressure conditions, the components of which are insulated from the corrosive and erosive effects of working fluids.